How to generate UUIDs in JS or React?

You're probably just fine generating them client-side - it takes many, many UUIDs before you even approach a 1 in a million rate of collision (citing the answer below).

This answer has several solutions to generating them client-side in a way that complies with RFC4122:

Create GUID / UUID in JavaScript?

Alternatively, you could generate temporary optimistic IDs and then replace them when you get a response from the server.

You can use the uuid npm package. You probably want to use a v4 UUID. (Previous UUID versions use things like a timestamp or network card MAC address as seeds to the random generator).